Membrane switches are well known and used in many different environments. One of the most prevalent uses of the membrane switch is in the keyboard of office machines such as typewriters, computers, workstations or terminals. One of the types of actuators most commonly used is illustrated in E. T. Coleman U.S. Pat. No. 4,528,431, and assigned to the assignee of this invention, which comprises a pivot plate and a wire coil spring attached to the pivot plate. This type of actuator gives the operator a reliable switch closure as well as a tactile feedback to assure the operator that switch closure has occurred. A significant portion of the cost of such a keyboard is the manufacture of the springs and the assembly with the pivot plate, in a manner to assure reliability.
In this type of keyboard, as the key cap of the keyboard is depressed, the coil spring compresses and at some point the compressed spring becomes unstable and catastrophically buckles. When the spring buckle occurs, the spring creates a moment and torques the attached tenon on the pivot plate to cause the pivot plate to pivot and to exert a force downward onto the membrane switch structure to deform the top layer of the membrane switch and close the contacts.
The spring disclosed in the Coleman Patent above and also found in Harris U.S. Pat. No. 4,118,611 and assigned to the assignee of this invention, possesses several desirable characteristics. These characteristics include a physical hysteresis in the switch activator, tactile feedback, audible feedback, snap action and the inability to get the switch to change state without a tactile or audible feedback (referred to as non-teasability). These characteristics are described in the foregoing Coleman and Harris patents.
With at least eighty key positions on a small computer keyboard and some keyboards having in excess of one hundred key positions, the combining of the spring and the pivot plate into a single item and the molding of the spring of the same plastic as the pivot plate presents a significant opportunity to reduce cost and improve reliability through elimination of parts, as well as eliminating the need to control assembly and manufacturing parameters.
The molded plastic spring provides, in addition to retaining the desirable characteristics of the wire wound spring of the prior art, an opportunity to improve the acoustic characteristics of the switch activator. The wire wound springs of the prior art produce a click type noise when the spring impacts against the frame of the switch actuation mechanism. Additionally, after the impact with the frame the spring will resonate producing a ringing or twang sound. The molded plastic spring does not produce the undesirable ringing sound while retaining the click sound which is desirable for the audible feedback to communicate to the operator, the transition of the switch.
A plastic molded spring and pivot plate, while more economical, requires entirely different functional control design considerations than when dealing with the characteristics of the wire wound coil springs. The control of the buckling action of the molded spring may be controlled by the physical design of the spring and by how it engages with other parts, where the wire wound coil springs did not present these opportunities.
It is an object of the invention to more reliably actuate the membrane switch with a device that is more economical to manufacture and assemble and which yields more consistent results.
It is an additional object of the invention to eliminate undesirable characteristics of sound of the prior art while retaining the desirable characteristics of sound for audible feedback found in the prior art.